(1) Field of the Invention
This invention generally relates to filters for electromagnetic interference and more particularly to filters for minimizing electromagnetic interference in ungrounded three-phase power systems.
(2) Description of the Prior Art
It is well known that electromagnetic interference (EMI) can have an adverse impact on the operations of electronic equipment. Typically switching actions on loads supplied from a power supply generate transients that produce the interference that is either radiated through space or conducted through power lines to the equipment. The problem is exacerbated when there is also a potential ground loop path for currents generated by such transients.
Moreover, the problem is frequency dependent. Although EMI can be present in conventional 60 Hz electrical systems, the problem intensifies at higher frequencies. For example, there are a number of applications where weight and size have been determining criteria for the selection of higher frequency power supply. Aircraft and ships are examples where, for purposes of size and weight, 400 Hz power supplies, rather than 60 Hz power supplies, are used frequently. Such power supplies can include motor generator sets or frequency changers that supply three-phase power through ungrounded delta- or wye- transformer connections. In these systems the production of non-linearities in the generated wave form can constitute still another source of EMI. Further, in shipboard and aircraft applications, metal generally surrounds the power system and related equipment and the metal can constitute a current path whereby any interference produces undesirable ground loop currents.
A number of approaches including the use of input and bypass filters have attempted to minimize the generation of this EMI and to isolate equipment from the effects of that interference. For example, U.S. Pat. No. 5,179,362 to P. L. Okochi et al. depicts a power line filter that attenuates normal mode noise between the power lines and common mode noise appearing between the power line and the ground line. The line filter includes a plurality of first inductors interposed between the live line and the neutral line, a first X-capacitor interposed between the live line and the neutral line on the load side relative to the first inductors, a plurality of second inductors interposed between the live line and the neutral on the load side relative to the first X-capacitor, and a Y-capacitor interposed between the live line and the ground line as well as between the neutral line and the ground line on the load side relative to the second inductors, the Y-capacitor constituting a common mode filter between the first inductors and the second inductors in response to common mode noise. The X-capacitor is connected directly between the live and neutral lines; the Y-capacitor comprises two capacitors in series between the live and neutral lines with a common point connected to ground.
U.S. Pat. No. 5,119,266 to Petry discloses an EMI filter protection circuit for use with an EMI filter in a three-phase power circuit that includes three capacitors coupled to the output terminals of the filter. The EMI filter and power circuit have a resonant frequency which is substantially equal to the line frequency. The capacitors of the protection circuit have values that are selected to change the resonant frequency of the EMI filter and power circuit such that the resonant frequency is not substantially equal to the line frequency. Essentially these capacitors are added in parallel to each leg of a three-phase delta transformer.
U.S. Pat. No. 5,083,101 to Frederick discloses an EMI filter including an inductor assembly for attenuating both common mode and differential mode interference. The inductor assembly includes an outer core of permeable material defining an outer window and an inner core of permeable material disposed in the outer window and defining an inner window. A first power lead for connecting the equipment to a source of electrical power includes a portion wound in one direction about only the outer core. The inductor assembly includes a second power lead for connecting the equipment to the source of electrical power with a portion of the second lead wound about both the outer and inner cores in the opposite direction to the winding of the portion of the first power lead. The outer core provides a magnetic path for the attenuation of common mode emissions and the inner core provides a magnetic path for the attenuation of differential mode noise. The filter can also include a multiple function capacitor including three capacitive elements.
U.S. Pat. No. 4,622,526 to Schneider et al. discloses a similar power line filter using a pair of inductors. In the Schneider et al. patent a printed circuit board has an extended grounding area to which are connected parallel capacitors, a pair of line inductors and a parallel resistor. Each inductor is formed as a multi-section coil wound on a plastic bobbin and two wound inductors are connected on rectangular cores to form line inductors. At least two of the capacitors are rectangularly shaped and are mounted on opposite ends of the line inductors. Electrically this circuit places an input capacitor and resistor across the input of the power lines, inductors in series with each line in an output capacitive network, much like the Frederick patent.
Another similar structure is shown in U.S. Pat. No. 4,342,013 to Kallman with capacitive input and output networks and inductors in series with each line. The filtering is accomplished by providing a distributive capacitance that is electrically connected between the power lines and the ground wire of a power distribution system. In addition to lumped parameter or fixed value capacitors, a distributive capacitance is provided by use of a printed circuit board upon which the electrical components forming the filter are mounted. A specially formed dual inductor is wound a common-core inductor using two toroidal cores preferably formed of ferrite each having a different permeability versus frequency curve. The filter is intended for use in three-phase Y-connected power distribution systems as well as conventional single-phase systems.
Filtering has been suggested through close coupling as shown in U.S. Pat. No. 4,760,356 to Kempster that discloses the power line filter having at least an inductive element and a capacitive element and a circuit network with a coil therein coupled to the inductive element. The circuit network includes a capacitive element and resistive element parallel connected and in parallel with a serially connected inductive element and coil. The coil is closely coupled with the inductive element of the power line filter.
In addition, and as described in more detail later, attempts to reduce EMI power supplies have included the use of bypass capacitors in the front end of the power supply that connect to electrical ground. Even when such filters are included in a power system, the levels of EMI can still exceed those which are acceptable.